To couple light from a single-mode laser into a single-mode step-index fiber, the spot diameter and the numerical aperture of the fiber must be matched to the laser so as to achieve minimum laser-to-fiber losses. The same applies analogously at the junction of two single-mode step-index fibers, where coupling losses are introduced if these fibers have different spot diameters and numerical apertures.
From an investigation by K. Shigihara et al, "MODAL FIELD TRANSFORMER BETWEEN DISSIMILAR WAVEGUIDES", ECOC 1986/pp. 185-188, it is known to provide a coupling unit between optical waveguides, lasers, and integrated optical units having waveguide profiles of different cross-sectional shape, namely rectangular and circular sections. To this end, a fiber of rectangular section whose cladding is made of so-called BK7 glass and whose core glass is doped with thallium ions is made. By heating a portion of this multimode fiber at 600.degree. C. for three hours, a diffusion process is caused by an ion exchange between the thallium ions in the core and the potassium in the cladding. As a result, the profile of the fiber core increases continuously and changes continuously from the rectangular shape to the circular shape.
In this manner, a multimode coupling unit between optical devices with a rectangular waveguide on the one hand and multimode fibers of circular cross section on the other is obtained, but it cannot be used for the above-mentioned optical single-mode elements and step-index fibers of the same, circular cross-sectional shape. In addition, such multimode coupling units must be subjected to a relatively long diffusion process.